Metals have a tendency to weaken, corrode, or crack in certain environmental conditions. This problem is prevalent in the oil and gas industry where extreme temperatures, pressures, and chemical environments are often encountered. Testing is often employed to determine what metal alloy is best under the well conditions and to also determine the lifetime of the selected alloy under those conditions.
Environmentally Assisted Cracking
Metal alloys have a yield strength which allows them to withstand a certain amount of yield stress with the occurrence of plastic deformation, but without cracking. Environmental conditions can significantly reduce the amount of force a metal alloy can tolerate, permitting the metal to fail significantly below the yield stress. This phenomenon is generally known as environmentally assisted cracking (EAC). EAC is induced by a combination of three factors: the presence of stress, a corrosive environment, and the sensitivity of the metal to corrosion.
Placing a stress on a corrosion sensitive metal in a corrosive environment comprised of substances such as halides and acidic substances can contribute substantially to EAC. The exact causes of EAC are not fully understood, but exposure to various environmental factors has been shown to be important, for example, temperature, pH, halide concentration, O2 concentration, and the presence of sulfur species such as H2S. Particularly, corrosive environments at high temperatures (e.g. >300° F.) can increase the susceptibility of a metal to EAC.
Individual metals and alloys react uniquely to environmental conditions. Therefore, it is important to test a metal under the conditions it will be used. EAC is a common problem in the oil and gas industry as corrosive chemicals and metals under high stress at extreme temperatures are frequently encountered in underground wells and deep sea drilling. In view of the expense of completing and producing a well, the metals and fluids used in these phases are carefully selected based on testing. Unexpected metal failures are both expensive to correct and potentially dangerous. Clearly, it is most important that such testing be conducted under conditions simulating those of the well.
NACE Tests
The National Association of Corrosion Engineers (NACE) has developed a standard test method (NACE TMO 177-96 Method C) for laboratory testing a metal's resistance to Environmental Cracking, EC. This type of testing is more commonly referred to as Environmentally Assisted Cracking, EAC.
The NACE method involves placing a stressed sample of the metal in a test vessel under corrosive conditions. The test specimen is a “C” shaped piece of material known as a C-ring. NACE standardizes the specific measurements of the C-ring.
The NACE C-ring includes two holes through which a bolt passes. The bolt is tightened on either end to apply a stress to the C-ring. To ensure that the proper amount of stress is applied to the C-ring, an electrical resistance strain gauge is used to measure strain in the C-ring at the time the bolt is tightened.
The bolt is tightened until the appropriate strain gauge reading is achieved. NACE specifies that the strain gauge is placed on the outside diameter at a point 900 opposite the axis of the bolt. After the tightening the bolt, both the strain gauge and the glue used to adhere the gauge to the C-ring must be removed. Once the strain gauge is removed, the C-ring and bolt are then cleaned and placed in the vessel for testing. NACE specifies that H2S is added to the vessel and the test is run for up to 720 hours with no provision for detecting early failure.
NACE also specifies that EAC is detected by removing the C-ring from the vessel and searching for cracking by visually examining the specimen. Since cracking cannot be observed while the test is in progress, tests often run much longer than required. Additionally, determination of alloy lifetime takes multiple tests since the test must be repeatedly stopped and restarted to determine if failure has occurred. Therefore, reliable real time observation of specimen corrosion damage is preferable.
Strain Gauge
As in the NACE method, strain on an object is an indicator of stress within the object. Strain is defined as the amount of deformation per unit length of an object when a load is applied. When a load is applied to a wire it undergoes strain, lengthening slightly. The strain causes a change in the electrical resistance of the wire. A strain gauge measures strain by measuring the wire's resistance.
Generally, the strain gauge is mounted to an object under strain with an adhesive and deforms with the object. The strain gauge is comprised of wires which stretch and change electrical resistance as the object deforms. A measure of the strain gauge resistance change correlates to the strain occurring in the object to which the strain gauge is attached.
Temperature is known to affect strain gauge measurements. The measured strain will tend to drift as the temperature of the strain gauge changes. The drift associated with the temperature of the strain gauge makes strain measurements at changing temperatures difficult to interpret. Therefore, it is important to compensate for temperature's effect on the strain measurement.
Background Methods
In general, the NACE method is used to test for corrosion damage in a C-ring specimen. The NACE apparatus allows the C-ring specimen to be compressed in an assembly. The force of the compression load is calibrated with strain gauges temporarily mounted on the outside surface of the C-ring arc. In the NACE method, the strain gauges are removed after calibration and are not used in the direct measurement of the corrosion damage. The strain gauges must be removed before the specimen is placed in the test chamber. After a prolonged testing period, the test is halted and the C-ring visually inspected to determine if EAC occurred. Unfortunately, with the NACE method, a real time reading of the strain in the C-ring assembly is not possible.
Several methods of real-time monitoring of strain in a metal component have been developed. In the previous methods of testing metal samples, real time monitoring of corrosion damage to the metal sample consisted of plotting strain measurements and visually monitoring the strain measurement to detect sample failure. Noise in the strain gauge signal and drift in the strain measurement due to environmental conditions, such as temperature, make it difficult to accurately determine when failure occurs based on the strain measurement.
Additionally, previous methods did not enable sensitive real-time monitoring of localized corrosion events such as pitting that often precede EAC, as well as the EAC event itself. Also, a means to determine the causative factors associated with the corrosive event, be it EAC, pitting, etc., by correlation to the time of the event's occurrence was not available.
Given the numerous variables that influence environmentally assisted cracking, sample corrosion damage, and variation of strain measurements, a method of measuring strain during the testing for corrosive damage to the metal sample that provides greater accuracy in corrosion damage measurement is desirable. A method which also provides the ability to monitor localized corrosion real-time and correlate the causative factors of corrosion with sample failure events is also desirable.